1. Field of the Invention
The present invention relates to an ion beam delivery equipment and an ion beam delivery method, which are used to produce and deliver ion beam of, e.g., proton or carbon ions to a tumor for treatment.
2. Description of the Related Art
There is known a treatment method for delivering ion beam, e.g., proton or carbon ions, to a tumor, such as a cancer, in the body of a patient. The ion beam delivery equipment for such treatment is comprised of a beam generator to produce the ion beam and accelerate it to a needed energy, a beam transport system, and a beam delivery nozzle. An ion beam accelerated by the beam generator reaches the beam delivery nozzle, which is installed in a rotating gantry to monitor and shape the therapeutic radiation field, through a first beam transport system and a second beam transport system, the latter being installed in the rotating gantry. The ion beam reaching the beam delivery nozzle is delivered to the tumor in the patient's body from the beam delivery nozzle. Known examples of the beam generator include a synchrotron (quasi-circular accelerator) provided with an extraction deflector for extracting the ion beam from the orbit (see, e.g., Patent Reference 1; U.S. Pat. No. 5,363,008).
In radiation therapy using an ion beam, e.g., with a proton beam delivering a radiation dosage to the tumor, by utilizing characteristics that most of energy of the proton beam is released just before protons come to rest, namely that a Bragg peak is formed just before the protons stop. The energy of the proton beam is selected to stop protons in the tumor so that the beam energy is released mostly to cells within the tumor or its microscopic extensions.
Usually, the tumor has a certain thickness in the direction of depth, i.e. along the direction of the ion beam, from the body surface of a patient (hereinafter referred to simply as “the direction of depth”). To effectively irradiate the ion beam over the entire thickness of the tumor in the direction of depth, the width of the Bragg peak must be spread out in the direction of depth. To obtain the required spread-out Bragg peak width, the energy of the ion beam is changed, that is, modulated.
From that point of view, a range modulation wheel (RMW) has already been proposed in which a plurality of blades each having a thickness varied step by step in the circumferential direction are set around a rotating shaft (see, e.g., Non-patent Reference 1; “REVIEW OF SCIENTIFIC INSTRUMENTS”, Vol. 64, No. 8, pp 2074-2084 and FIGS. 30 to 32, in particular, p 2077 and FIG. 30 (August 1993) and Non-patent Reference 2; “PHYSICS IN MEDICINE AND BIOLOGY”, Vol. 48, No. 17, pp 2797-2808 (Sep. 7, 2003)). The plural blades are mounted to the rotating shaft. At the time when the ion beam passes through the blade, the energy of the ion beam is attenuated more as the ion beam passes through the blade having a larger thickness, and therefore the Bragg peak is produced in a portion of the tumor near the body surface of the patient. With the rotation of the RMW, the position in the direction of depth where the Bragg peak is formed varies cyclically. As a result, a Bragg peak width comparatively wide and flat in the direction of depth of the tumor can be obtained, looking at the beam energy integrated over time. Further, it is known that the SOBP (Spread-out Bragg Peak) width can also be formed by using a ridge filter (see, e.g., Non-patent Reference 1; in particular, p 2078 and FIG. 31).
The dose irradiated to the tumor can be determined through steps of detecting charge ionized in a dose monitor, and converting the detected value into a value of the actually absorbed dose by employing the conversion coefficient. The dose monitor is installed upstream of the patient along the beam path. Then, it is suggested that the coefficient for conversion between the detected value measured by the dose monitor and the value of the dose actually irradiated to the tumor is correlated to the beam penetration depth and the SOBP width (see, e.g., Non-patent Reference 2).